In the last few decades, the rapid growth in world population and industrial development has lead to massively increased usage of fossil fuels such as coal, petroleum and natural gas and the resulting formation of carbon dioxide; moreover, due to the deforestation and the reduction of rain forest, the dynamic equilibrium of carbon dioxide formation and conversion has been seriously destroyed. Consequently carbon dioxide content in the atmosphere increases year by year; the seriousness of global warming attributed to carbon dioxide emission has increased, and the potential dangers to humanity have driven many countries to research the reduction of carbon dioxide.
The reduction technology of carbon dioxide currently can be divided into two methods, physics-based and chemistry-based. For physics-based methods, carbon dioxide is captured from atmosphere and then stored underground or under the sea bed using high pressure compression. From the viewpoint of equilibrium and cycling of carbon dioxide on earth, the amount of carbon dioxide has not been reduced; hence, the use of chemistry-based methods to convert carbon dioxide into useful materials has become the core of carbon dioxide reduction technology. However, although several chemistry-based methods have been developed for the conversion of carbon dioxide, these chemical processes have the following limits: First, since carbon dioxide is very chemically inactive, catalysts must be used for conversion reactions, but catalysts are very expensive and the reaction lifetime is limited; Second, since carbon dioxide and its counterpart molecules are usually in different phases at room temperature and atmospheric pressure, the reaction must be carried out under high temperature and high pressure environments; Third, such long reaction times are required for the chemical reactions, that the reaction times can be several hours to several days depending on the types of catalysts; all the above mentioned issues have limited the massive demands for carbon dioxide conversion in industries. Moreover, such chemical processes are not suitable for household applications.
Based on these considerations, a plasma-based technology is presented for carbon dioxide conversions. When molecules enter into electric fields, they are excited and ionized by collision with accelerated electrons to generate various species such as atoms, electrons, ions, free radicals, etc. The mixture of these species is plasma. These activated species generated by plasma bombardments can recombine to form new products. The molecules used in this plasma processes do not have to contain chemically active groups, such as C═C bonds. As compared to the complicated processes and steps in conventional chemical syntheses, the plasma process is simple and fast. Besides, no solvent needs to be used and the hazard to the environment is greatly reduced; moreover, mass production can be easily reached to satisfy economic efficiency in industries. Besides, since plasma can be initiated in a simple device, it can thus be miniaturized to apply in portable or mobile commercial products, which will be a great advantage for the plasma technology to be extended to more applications.
Some studies have been aimed at the reaction mechanism of carbon dioxide in plasma. Carbon dioxide consists of two strong covalent bonds with low chemical activity; the conventional synthesis of carbon dioxide has to be induced only using catalyst activation. Buser et al. (J. App. Phy. 41, 472, 1970) found that carbon dioxide in plasma can be decomposed into carbon monoxide through vibration excitation. It was reported that as carbon dioxide is decomposed via the anti-symmetrical stretching mechanism, the initial energy is 0.1 electron volt (eV) and the energy required to overcome the band gap is 5.5 eV. This energy is smaller than the direct dissociation energy of C═O bond, which is about 8 eV. Therefore, via plasma activation, the bond dissociation of carbon dioxide can be achieved at a lower energy level, and the subsequent recombined reactions can be carried out. Besides, the past studies reported that the carbon dioxide can be converted into hydrophilic functional groups by plasma activation, such as carboxylic acid or alcohol, etc. When such derivative functional groups from carbon dioxide are attached to the material surface, hydrophilic properties of material surfaces can be enhanced.